Motor vehicle air suspension systems utilize compressed air operated leveling devices, as for example air springs and/or air spring over shock absorber modules or a combination thereof, to provide ride and leveling control of the vehicle. Such air suspension systems utilize an air compressor to provide a source of compressed air to the air operated leveling devices. In a typical configuration, as for example described in any of U.S. Pat. Nos. 4,829,436, 5,465,209, and 6,698,778, the air compressor is selectively connected by electronically controlled solenoid valves to the air operated leveling devices, a compressed air reservoir (optional), an air intake, and an air exhaust. Most air suspension systems operate in an “open state” in the sense the excess air volume within the system is vented to the atmosphere at the exhaust and the source air for the compressor is drawn from the atmosphere at the intake; however, at least one air suspension system (see above cited U.S. Pat. No. 6,698,778) operates in a “closed state” in the sense that air is not exchanged with the atmosphere, wherein excess air volume is stored in an air reservoir and the source air for the compressor is either the air reservoir or the air springs.
Turning attention now to FIG. 1, an example of a prior art motor vehicle suspension system 10 is depicted, as generally also shown and described in aforementioned U.S. Pat. No. 4,829,436 to Kowalik et al, issued on May 9, 1989, the disclosure of which is hereby incorporated herein by reference.
The motor vehicle air suspension system 10 includes four compressed air operated leveling devices 12 which may be air springs and/or air spring over shock absorber modules, or a combination thereof, a computer 14, a compressor/exhaust apparatus 16, an air drier 18, a pressure switch 20, a valve assembly 22, a plurality of air lines 24 and signal lines 26. The plurality of air lines 24 go to the four leveling devices 12 to provide pressurized air from the valve assembly 22. A road wheel 28 is associated with each leveling device 12. The computer 14 receives an ignition signal, vehicle speed signal and vehicle door disposition signal. The computer 14 controls the operation of each solenoid valve in the valve assembly 22. The computer 14 also receives input from four position sensors 32, one at each of the four road wheels 28 through the four signal lines 26. The compressor/exhaust apparatus 16 selectively sources or vents air through the air drier 18. A master air line 30 runs from the pressure switch 20 to the valve assembly 22 which controls compressed air communication between the compressor/exhaust apparatus 16 and the individual leveling devices 12 in response to signals from the computer 14. The pressure switch 20 is optional, and is used to monitor the air pressure at each air leveling device 12.
When the engine of the motor vehicle is shut-off, the temperature within the air springs may be considerably higher than the ambient temperature, additionally, the vehicle may be loaded or unloaded shortly after the engine shut-off event (as for example presumed to have concluded within about 30 minutes, or so). As a result of these conditions, the location of the sprung mass (i.e., the vehicle body) relative to the location of the unsprung mass (i.e., the wheel), referred to herein as “trim height”, may vary when the temperature in the air springs has arrived at equilibrium with the ambient air temperature.
Accordingly, what is needed in the prior art is some methodology to predict whether an undesirable vehicle height may occur after engine shut-off and provide execution of a remedy therefor if the prediction so warrants.